In typical point of sale systems for retail stores, a set of special purpose computers are positioned at the retail check-out locations. These are locally networked with a server located at the site of the retail store. For many retail stores, this requires a level of system maintenance responsibility that is beyond the staff capabilities of the retailer or too costly. In this invention, the point of sale system is designed so that it is provided as a service shared among participating retail vendors. With this kind of an architecture, the invention provides the service security, data integrity, robustness and redundancy.